Oil spills in the ocean cause a major threat to environment and is a serious problem. Failure to control the spilled oil can lead to contamination of miles of shorelines creating adverse impact on the flora and fauna and invariably on the human life. Similarly, the leakages of hazardous liquid chemicals in laboratory or in an industrial setting can also be equally dangerous and therefore need to be wiped out completely. Further, the water bodies such as lakes, rivers, ponds as well as soil in agricultural lands contaminated with oils, lipophilic substances, other toxic chemicals also pose an environmental threat and has to be addressed to control the pollution.
There are many ways to remove oil or other chemical spills such as use of surfactants, dispersants, liquid soaps or use of activated carbon. The use of these technologies have certain limitations such as the treatment with activated carbon in granular form is confined within the vessel in the form of bed which might create impedance to the flow of water, the granules may also trickle down in the water and there is a need for further filtration which requires the use of suitable membranes that makes the process costly. The use of liquid soap is limited as it is unable to effectively remove any form of the contaminant. The effective use of surfactants and dispersants depend on the environmental factors including the salinity and temperature of water, nature of oil spilled etc.
Absorbent foams are the alternative techniques used in the removal of pollutants. The polyurethane foams are widely used in the removal of oil, however, the hydrophobic nature of the foam restricts its use to removal of hydrophobic liquids from the mixture of hydrophobic and hydrophilic liquids. Further, the polyurethane material is often attacked by oil causing swelling of the material resulting in drastic impairment in the mechanical properties. Modification of hydrophobic foams by coating or inserting lipophilic or hydrophilic material is known in the art, however, the materials lack the mechanical stability and limits its use.
Macroporous sponges with high surface area and interconnected pores have attracted much attention recently for removing oil or hazardous chemicals from the surfaces of water bodies. An article titled ‘Evaluation of hydrophobic polyvinyl-alcohol formaldehyde sponges as absorbents for oil spill’ by Pan Y, Shi K et. al published in ACS Appl Mater Interfaces. 2014 Jun. 11; 6(11):8651-9 discloses macroporous and hydrophobic polyvinyl formaldehyde (PVF-H) sponges prepared by the reaction of stearoyl chloride with hydroxyl groups of hydrophilic PVF sponge at different temperatures. The as-prepared PVF-H macroporous sponges have interconnected open-cell structures, with the average pore size from 60 to 90 μm and porosity greater than 94.8%. Such PVF-H sponges absorb oil products such as toluene, n-hexane, kerosene, soybean oil, hydraulic oil, and crude oil up to 13.7 g·g (−1) to 56.6 g·g (−1).
Article titled ‘Evaluation of Electrospun Polyvinyl Chloride/Polystyrene Fibers As Sorbent Materials for Oil Spill Cleanup’ by Haitao Zhu et. al published in Environ. Sci. Technol., 2011, 45 (10), pp 4527-4531 disclose high-capacity oil sorbent consisting of polyvinyl chloride (PVC)/polystyrene (PS) fiber prepared by an electrospinning process. The PVC/PS sorbent exhibits high absorptivity of organic solvents such as motor oil, peanut oil, diesel and ethylene glycol.
U.S. Pat. No. 5,002,984 discloses an insolubilized polyethyleneimine (PEI) polymer deposited in situ within an open celled cellulosic sponge. The insolubilized PEI is prepared by initially forming a water solution of PEI and an insolubilizing co-reactant, applying said solution to a cellulosic sponge, drying the treated sponge to remove water of solution, followed by curing the sponge at temperature in the range of 130° C.-165° C. The cellulosic sponge of US'984 is used for the water treatment as ion-exchange resin.
U.S. Pat. No. 6,306,488 relates to an open-celled sponge having a cellulosic component of large pore size and a polyvinylchloride component of small pore size for use in water filtration to absorb toxic heavy metals, sound absorption and thermal insulation. The average pore size of the coarse pore grade is about 2 mm and the average pore size of the fine pore grade is about 1 mm. The average diameter of said interstices ranges between 0.2 and 20 microns and the surface area of said microporous PVC is between 0.5 and 12 square meters per gram. The microporous PVC is produced by forming a mixture of a colloidal dispersion of a latex of a non-film forming PVC with a water-soluble organic non-solvent for the polymer, and subjecting the mixture to thermal curing. The mixture is then added to the cellulose sponge in dry form in controlled manner to attain proper saturation of the cellulosic sponge. The impregnated sponge is further subjected to heat treatment at temperatures in the range of 220° F. to 280° F. for a period of from 15 to 100 minutes to cause the dispersed polymer particles to cohere or sinter together, thereby forming a microporous structure within the pores of the cellulosic sponge. From the disclosure in said US'488, it is observed that the process to prepare the said sponge with desired characteristics varies with heating conditions and quantity of PVC introduced in the sponge.
Thus, there remains a need in the art to provide macroporous polymeric sponges that can absorb both organic fluids as well as water. For preparing macroporous absorbents, it is necessary to control the shape, size and interconnectivity to facilitate mass transport through the macrochannels with limited pressure drop. The methods disclosed in the art are tedious, require the use of costly chemicals and need selective conditions to be maintained throughout the process to obtain the specific architectures of the sponges. Thus a rational approach to provide polymeric sponges with desirable growth morphologies and pore structure is still a need of the art.
Ice-templating or freeze casting method has seen a surge of interest over the last decade. It is mostly based on physical interactions and mechanisms and the materials can be templated independently of their nature. The method involves the use of ice crystals as a template for the macro structures, with the ice-crystals being formed during the unidirectional freezing of an aqueous suspension containing the organic or inorganic solid particles that need to be assembled in the macrostructure. The ice-templating technique can be used to simultaneously control several different sizes in the material.
The commercially available absorbent material called Solusorb which is chemically activated charcoal is used in research laboratory or in industry to mop up spills. The material can absorb ˜220 ml/kg of the solvents.
The present inventors observed that the absorbent materials known in the art and the process for preparing the same have certain shortcomings which need to be curtailed so as to provide a more versatile absorbent material i.e. sponges which have great affinity to both water as well as organic compounds, with enhanced absorptivity, useful for the control of spills/leakages. They were of the further opinion that the solvophilicity of the sponge can be tuned by grafting different chemical moieties on to the ice-templated hydrophilic polymeric material.